The present invention refers to a protection device for electrical appliances, in particular electric motors compressors and transformers.
For the sake of simplicity of the treatise, in the rest of the description we shall refer in a non-limiting manner to electric motors.
As it is known, in order to ensure that electric motors have good operation and last a long time, it is necessary to protect them from overloading that is caused by over-current that can pass through the windings of such motors and the consequent overtemperature that can affect the windings themselves.
Such overloads can substantially occur during the starting phase of the motor or during its steady state operating.
In any case, it is currently known to arrange, in one or more areas of the windings, one or more protection devices that are suitable for selectively connecting the winding to which they are connected to the power supply line.
Generally, in order to protect the windings of the stator and/or of the rotor of an electric motor from overtemperatures, thermostats are used that are configured so as to disconnect such windings from the electric power supply in the case in which the temperature perceived by the thermostats themselves exceeds a threshold value that is considered dangerous for the integrity of the motor.
Such thermostats, however, are not able to ensure a rapid interruption of the power supply in the cases in which the temperature of the windings grows very quickly like, for example, when starting up a motor.
Indeed, in such cases, the room temperature perceived by the thermostats known today due to their thermal inertia is not capable of keeping up with the rapid increase in the temperature of the windings thus resulting to be lower than the latter. In such a way, the disconnection of the supply line can occur when the windings already subjected to overtemperature have been damaged or become burnt.
In order to avoid this drawback or rather to compensate for the thermal inertia of the thermostats, it is possible to use protection devices that are also sensitive to electrical current in which the heat generated by the passing of current by Joule effect is equal to the difference between the temperature of the protection winding and the temperature that is perceived by the thermostat.
The sensitivity to electrical current makes it possible, in particular, to obtain an optimal protection of the motors in the rotor blocking conditions in which room temperature is normally lower than that of the windings.
For such a reason protection devices are known that are capable of disconnecting the electric appliance from the power supply not only when the temperature exceeds a higher temperature than the threshold value, but even when the current exceeds a greater current threshold value.
Such known protection devices generally comprise a fixed metal support for a stationary electrical contact and a bimetallic blade that is provided with a movable electrical contact, which are arranged so that the electric contacts make a switch that is normally closed.
The bimetallic blade used, in detail, is made up of at least two metals having different thermal expansion coefficient and it is shaped so as to snap into a position that is distant with respect to the metal support, in which the electric contacts are disconnected and the switch is thus open, when the temperature it is subjected to exceeds a higher temperature threshold value.
The bimetallic blade is capable of returning into the contact position closing the switch when the temperature it is subjected to falls below a lower temperature threshold value.
In order to make the aforementioned protection devices sensitive also to very small current values, it is known to connect, to the metal blade, an additional heating device i.e. a resistance that is arranged so that the blade itself is sensitive to the heat that is spread by the additional heating device. Indeed, in such a way the heat that is produced by the resistor after even very small current has passed is such as to increase the temperature around the blade beyond the higher threshold value.
The aforementioned protection devices have some drawbacks.
A first drawback lies in the fact that the connection of the resistance of the additional heating device is generally carried out by means of a welding operation, which makes the assembly of the aforementioned protection devices complex and costly.
A second drawback is the fact that such protection devices cannot be easily adapted to the different application scenarios for which they are intended due to the fact that, in order to modify the sensitivity to the temperature/current of the device, it is necessary to change the resistance of the additional heating device by subjecting the bimetallic blade to new welding operations.
Another drawback lies in the fact that the behaviour of such protection devices cannot always be repeated; indeed, the sensitivity of the bimetallic blade to the temperature varies as the distance the resistance of the additional heating device is arranged at varies. This distance can vary from one device to another within a range of values that is set based upon the tolerances of the mounting procedure.
Moreover, it is worth underlining that the electric contacts are subjected to strong wearing following the impact of the blade on the metal support every time the switch passes from the opening position to the closing position. Such wearing, therefore, limits the durability or rather the average life time of the protection device.
One example of a known protection device is described in the patent application EP 0 090 491.
Such a protection device comprises a metal casing that is open, at the top, on the bottom wall of which a bimetallic blade is welded to and that comprises an end portion that is suitable for being used as a terminal. The casing is coupled at the opening with a metal plate that substantially closes the casing itself. Such a metal plate comprises a first end portion that is suitable for being used as a terminal, an intermediate portion that is suitable for making the additional heating device and a second end portion that is suitable for supporting a stationary electrical contact. Between the metal casing and the metal plate there is an insulating gasket that wraps around the lateral portion of the metal plate for the entire length of the plate itself so as to prevent it from coming into contact with the metal casing causing a short-circuit.
In this case the metal plate and therefore also the additional heating device always remain in contact with the insulating gasket. The heat, therefore, is dissipated not only by the additional heating device but also by the insulating gasket which, on the other hand, is always colder than the heating device itself; moreover, the maximum temperature of the additional heating device must be kept below the maximum temperature allowed by the insulating gasket in order to prevent it from becoming burnt. Such circumstances, therefore, represent an obstacle to an efficient heat exchange of the additional heating device towards the blade i.e. to a condition that is essential for a high sensitivity of the protection device.
A need therefore exists for a protection device for electrical appliances, in particular electric motors, compressors and transformers, that has high sensitivity even with low current values and a behaviour that is highly repeatable.
A need also exists for a protection device for electrical appliances that makes it possible to change its sensitivity to temperature and/or to current in a rapid and simple manner.
A need exists for a protection device for electrical appliances that is simple to assemble and that is less expensive with respect to known devices.
A need exists for a protection device for electrical appliances that has an average life time that is greater with respect to known devices.